The invention relates generally to a multi-layer foil such as a hot embossing foil.
In an effort to improve the safeguards against forgery of documents such as check cards, credit cards, savings books and similar security documents and items, such documents may be provided with a foil such as a hot embossing foil, which has a three-dimensional patterning thereon. For example, such a patterning may be a structure which has an optical diffraction effect, such as a hologram, by means of which it is possible to produce quite definite optical effects, whether by radiation with natural light or by radiation with artificial light of a particular wavelength. In order to forge security foils of that nature, it is necessary for the forger to remove the patterning with the utmost degree of accuracy, and to transfer it on to the forged foil. However, the patterning is generally covered by a layer of lacquer to protect and safeguard it, and therefore the layer of lacquer must be removed in order to be able to take off the patterning for transfer thereof.
For example, U.S. Pat. No. 4,469,725 discloses an identity card which is formed from a laminate and which comprises two layers with different levels of optical transmission or of different colours. The adjacent surfaces of the two layers are provided with complementary and mutually interfitting impressions and raised portions respectively. In the course of manufacture, the two layers are pressed against each other, at increased temperature, whereby the layers are welded together in the interface region thereof, thus producing a unitary card. In that card, the recesses and the raised portions on the respective layers are comparatively high, corresponding to the thickness of the respective layers, more specifically being such that, when the two layers are brought together, the recesses and the raised portions engage one into the other. For the purposes of manufacture therefore, the layers are individually provided with the raised portions and the recesses respectively, and are then laid one upon the other in such a fashion that the raised portions and the recesses fit into each other. It will be appreciated that such a procedure is not possible when producing embossing foils, by virtue of the thickness thereof being small in comparison with the thicknesses of the layers of the known card. Furthermore, structures which produce an optical diffraction effect are so fine that in practice it would never be possible to provide for accurate location of two separate layers relative to each other, in such a way that surface configurations which are adapted to match to each other could be brought into interfitting relationship.
Hitherto, when producing hologram structures in embossing foils, the procedure involved was generally such that a metallised layer was produced, and the hologram or other structure producing an optical diffraction effect was then produced therein. The metal layer bearing the above-mentioned structure was then covered by a layer of protective lacquer. The lacquer used to produce the protective layer is one which on the one hand has very good adhesion to the metal layer and which on the other hand can only be dissolved away with very great difficulty, using conventional solvents. Nonetheless, persons attempting to forge such documents have repeatedly succeeded in exposing the surface of the patterning on the foil by removing the layer of lacquer.